Optical feed-back densitometer



Filed April 4, 1946 LINEAR AMPLIFIER @kwam Patented Aug. 25, 1953 UNITED STATES PATENT OFFICE OPTICAL FEED-BACK DENSITOMETER Monroe Hamilton. Sweet, Binghamton, N. Y., as`- signor to General Anilinc" & Film Corporation, NewYork, N. Y., a' corporation of'l Delaware Application April 4', 1946, Serial No. 659,457

3 Claims.

This invention relates` to logarithmically re- 4sponsive measuring circuits, and more particularly to circuits including feedback means controlling the output of an energy emitting device in response-to the outputof an energy. sensitive.

element receiving incident energy from the device.

In numerous-mensuration operations, an object is to obtain a direct indication of values which 'are logarithmic functionsoi the measured quantity. Atypical example isi the measurement of the density ofaphotographic lm. In the usual measuring instrument for determining the value of the nlm density, the light flux transmitted throughv the film is directed upon a suitable photo sensitive device, such as a photoemissve vacuum tube. The output of the phototube'is then'amplifiedA and thev relative vvalue thereof indicated upon a suitable current measuring meter.` Theradiant' fluxY incident upon the phototube is an inverse logarithmic function of the density of the film. If the output of the phototube is linearly amplified, the indicating meter must be provided with a scale which isl graduated logarithmically, if it is desired to ready density directly on the meter. Asis known; a logarithmic scale is non-uniformly graduated,- theindica thereof being-crowded near one.V end of the scale and being widely spaced near the-opposite end thereof. The non-uniformity of the scale graduations adversely affects theaccuracy and facility of the meter readings.

Various expediente haveY been proposed for obtainingfdirect indicationsv of. density.- on-a meter having a substantially uniformly graduated scale; Amongother expedients, cut pole pieces have been used in the meter to vary the sensitivity of response thereof over different. portions of the scale. The results obtainable by such expedients have been generally unsatisfactory. In my copending application Serialk No."f452,697, led July 29, 1942, for Direct Reading Densitometer, now issued as U. S.; Pat. 2,406,716, there is described and vclaimedsfa suitableelectronic Ymeasuring. circuitV for obtaining direct readings upon a:uniformlygraduatedmeter scale of thedensity of'a photographic film. In thiscircuit, a logarithmic amplifier isprovidedbetweenthe output of a.l phototube andthe. indicatingmeter; The parameters of the ampliier Iare so selected that the meter indicates directly the. density of the sample upon-*a uniformly graduated scale. The light ux incident upon the phototube is an inverse logarithmicfunction of the density of the sample', and theoutput. current of the phototube isa direct: functionk of` the lightlincident there- 2l upon. By interposing a logarithmically responsive amplifier between the phototube and the metensuch compensation is effected that the re1- ative currentowing through the meter becomes a directfunction of the. density of the sample.

The described circuit has had very satisfactoryv commercial use.

In the above described direct reading densitometer, the amplilier parameters must be so selected that the amplifier is operating at values such that the grid potential is a logarithmic function of the grid current. It is in some respects desirable tor employ a conventional amplifier where the grid potential or the amplifier response is linearly related. to theV phototube current. Accordingly, the. present invention comprises a measuring circuit in which although a linear amplifier is used. direct indication is obtained of logarithmic values the necessary compensation in the indication is effected by controlling the luminous excitation of the light source in accordance Withthe response of a photoi-emissive vacuum-tube or. similar photo-sensitive elements.

It is, therefore. amongA the objects of this invention to provideA a simple, logarithmically responsive. measuring circuit; to provide such a circuit including a linear amplifier` controlling the energization` of an energy emitting element responsive to the output of an energysensitive element receivingV incident energy from the energy emitting element; to provide a logarithmioally responsive measuring circuit including a light source, a photo-emissive vacuum tube .arranged in operative relation therewith, and a linear amplifier controlling the energization of the light source responsive. to the output of the photo-emissive vacuum tube.; and to provide such a logarithmically responsive measuring Hcircuit in Which the indicating means may be connected either' in the output circuit of the amplier or in the input circuitiof the light source.

These and other'obiects, advantages `and novel features of the invention will be apparent from the following description and the accompanying drawing. In the drawing:

Fig. 1 is a schematic diagram illustrating the operationof the measuringk circuit ofthe present invention.

Fig. 2 is. a schematic circuit diagram of one embodiment of.r the invention.

Fig. 3 is a schematic circuit diagram of Ianother embodiment-0f the invention..

According tothepresentinvention, a radiant flux sensitive:y element is arranged inoperative relation to receive flux from a radiant flux emitting device.

The light sensitive element of the type used herein has such characteristics that its electrical output is a linear function of the incident light radiation. When the arrangement is used to measure a logarithmic function, for example the density of a sample interposed between the light source and the photo-electric tube, the photo tube current is amplified and the output of the amplifier is utilized to control variable resistance means which in turn controls the luminous excitation of the light source. In this manner a feedback loop is established between the linearly amplied output of the photo tube and the source from which excitation of the light is derived.

The light source employed comprises a lamp which converts electrical energy into radiant energy and has such inherent characteristics that the radiant energy bears a logarithmic relation to the electrical energy supplied to the lamp; in other words the candle power output thereof varies approximately logarithmically as a function of the electric current or voltage applied to the lamp. Vari-ous types of incandescent lamps are known to possess this desirable characteristic.

It was mentioned before that in determining the density the radiant flux incident upon the photo tube is an inverse logarithmic function of the density of the film. Consequently, when this circuit is used for density measurements the amount of light reaching the photo tube from the lamp through the sample varies inversely with the logarithm of the density. The measuring instrument connected directly to indicate the photo tube current would require -a logarithmic scale unless provision is made for changing this indication, that is for correcting the indication in accordance with a logarithmic compensator. Instead of varying the indication of the meter anywhere in its energizing circuit the light intensity which excites the photo tube is caused to vary in such manner as to compensate for the linear response of the photo tube and provide a logarithmic response in the system. With this compensation in the circuit response, the indicating meter may have a standard, a uniformly graduated scale to give direct readings of the density of the sample interposed between the lamp and the photo tube.

Referring to Fig. 1 of the drawing, the invention is illustrated, by way of example, as incorporated in a direct reading densitometer in which a uniformly graduated scale meter provides direct indications of the density of a sample being examined. For this purpose, light from a suitable electric lamp I, which is preferably an incandescent lamp, is concentrated by a condenser lens II and directed, through a suitable filter I2 and `.a sample I3 mounted on a support I4, upon the cathode I6 of a photo-emissive vacuum tube I5. Cathode I6 and anode I'I are connected by conductors I8 and I9, respectively, to the input of a linear amplifier 2B. Amplier 20 is supplied with operating potential from a suitable source of preferably constant direct current potential not shown here, connected to terminals 2 I. The amplifier includes means for controlling the illumination of lamp I6, in a manner described more fully lhereinafter, and for this purpose the lamp Ill is shown connected to the amplifier by means of conductors 22. A suitable indicating meter 25, is actuated by the amplifier 2U, and is shown connected thereto as indicated.

The operation of the arrangement is as follows. The density of sample I3 is an inverse logarithmic function of its light transmission. The transmission, in turn, is a measure of the amount of 4light reaching phototube, I5 from source I0 through the sample with a constant intensity of the source. From the data relating lamp current or voltage applied to its terminals to relative lamp candle power, it can be shown that the candle power is a logarithmic function of the applied voltage, insofar as respects an incandescent lamp (see Weaver & I-Iussong Note on the Color Temperature- Candle Power Characteristics of Tungsten Lamps, J. O. S. A. 29, 17 Jan. 1939). Accordingly, if the operating potential applied to light source Iii is modified as an inverse function of the output current of phototube I5, the candle power of source Iii will be varied logarithmically as an inverse function of the phototube output current. Consequently a suitable current or voltage indicating means connected either in the output circuit of phototube I5 or the energizing circuit of source I0 will directly indicate the density on a uniformly graduated scale.

In effect, the intensity of lamp IB is controlled in such a manner that at low densities (i. e. high light transmission values) the light flux incident upon photo tube I5 is a minimum, and as the density of sample I3 increases (i. e. less light is transmitted), the intensity of lamp Ii) is also increased. With such arrangement, it is possible to obtain a uniform response of meter 25 to the density of sample I3.

If meter 25 is calibrated to read densities from 0.0 to 3.0, which is a useful range, and with high linear amplification of the output current of photo tube I5, the following relationships apply:

M c-FU where Fc is the lamp intensity as automatically corrected, or modulated, to obtain direct readings of density upon the substantially uniformly graduated scale of meter 25, M is the relative meter response and FU is the relative light flux which would be transmitted if the lamp intensity were not modulated. Furthermore,

FU=anti10g (3.0-D)

where D is the density of sample I3 and unit flux is assumed for the lamp at density 3.0. From these two equations, it will be apparent that M *antilog (3.0- D) in other words, in order to obtain uniform meter response for differing density values of sample I3, the corrected lamp intensity must vary directly as the meter response and inversely as the antilog of the total meter density reading minus the density of sample I3. As explained above, this is accomplished in the present circuit by linear modulation of the operating potential applied to light source I0, which results in logarithmic modulation of the candle power or flux output thereof.

Fig. 2 represents a specific embodiment of an arrangement for obtaining the results of the circuit of Fig. 1. The operating potentials are derived from a suitable source of substantially constant direct current potential connected between terminals V2| to a potentiometer or Voltagez divider. 3B... Cathode, IB of photo; tube leisf connected through. conductor. I3;to1 a tap ,r 25; onv

potentiometer.` 30; Conductor l9lconnects1anode.

I'I to the control gridZF! of;angelectroniciamplier tube 3,51 Cathodeof'. tube135f ispconnected to a tap 3| ofpotentiometergii. Anodeor plate 32' of tubeA 35 is connected through, a.v suitable load resistor 331and, inV series. the indicat-- ing Vmeter 25 to tap 25S;

The load' resistor 34 of' the photo tube; l5. is

also utilized as the inputcircuit origridresistanceV for the amplifier tube 352.' A minimum bias' voltage for this tube.isnderivedfromithe voltage.l

divider between cathode 2B." and tap. 3.5. The

ohmic value of' resistor 341 is preferably'high in; the neighborhood of 20.1negohms'inforder to in sure a satisfactory voltage dropzfor biasing the amplifier tube 351 when the photo'. tube is= conducting.

'With the describedarrangementg: as "the lightv falling-.upon phototubel iifrom .source` lilithrough sample I 3" increases, the phototubeA output current increases. This` current produces axvoltage drop across resistor 34' driving thev control; grid 21' more positive whichincreases the conductivityA of tube 3,5'. The increase orA decrease inthe formerV 45 andi control tube 50.' Conductorsv 22 connect the secondarywinding 13.3` of transformerl 45 tothe filament of` lamp I0.' Ina specific example, transformer Mi'raises the. potential of the supply line to 560 volts in ordery toprovide suflicient anode voltage for the control tubev 55. Transformer 45.' reduces the voltage applied to the primary- Winding 42 to about six volts, required for the lamp ID which has preferably a six candle power rating.

The secondarvvvinding 4|' of transformer 4i) and the primary winding4 42 of transformer 45.

are in series and the free terminals thereof .connect' to theV anode '4S' and' cathode, 44 of the. control tube 55), respectively. The circuit.. of the controlV grid`l is completed to the cathode 44 through the anode load resistance 33S of.` the ampli-Iier tube 35" in series with the indicating meter 25. The screen grid il'is not usedlin this particular arrangement; and therefor is connected to the anode 46 and the tube 55 is actually operating as a triode.

As can be seen from the circuit the eective anode voltage for the tube 50 is derived from the secondary Winding 4I of the transformer 40, whereas the grid voltage is derived from the current flowing through the load resistor 33. When there is no current flowing in this resistor the grid 5i has zero bias and is effectively at cathode potential. This condition calls for maximum anode current of tube 50 providing a maximum energy transfer from the A. C'. source to the exciter light le. Any variation in conductivity of the tube 35 will cause corresponding current variations in the load resistor 33 resulting in a corresponding grid voltage for the tube 50. This voltage is negative with respect to the cathode c4 inasmuch as the conductivity of the ampliiier tube 35 impresses on the grid 5| a potential more negative than-1 the:l cathode' 44'. Current? variations of the tube 35 as pointedt out above` depend-on the light` intensity reaching thephoto cell and is a direct function thereof. These .variationsimpressedion'l the control tube 5l will cause variations in the light intensity of the exciter lamp. l @inasmuch as thecurrent in the primary Winding 43; depends on the. conductivity ofthe tube 5E). n view ofthe factthat the characteri'stics of the exciter. lamp E0 are such that variations of filament excitation produces a logarithmicivariation of. candlepower output the light.` energy impressed on the photocell will vary logarithmically With respect to the linear variation of energizing voltage of the lamp l0.

As set forth above, the anode potential of the tube'35 varies-inversely. with the output current of phototube I5, and thusI inversely as Ia linear function of the light. incident upon the latter fromlamp I0 through sample I3. Accordingly, the illumination of lamp I0 is varied as an inverse logarithmic function of the light incident upon phototube l5'. Meter. Zameasures the current iiow inthe output circuit of ampliiier 35, and thus indicates directly, on auniformly graduated: scale, the. clensity'ofA sample I3 as measuredby phototubeV i5, in accordance with the relations between compensated lamp intensity and` incident` flux versus meter reading set forth above.

Fig. 3 represents another. embodimentv of the invention Whichdiiers fromithat shown in-Fig. 2 only inthe locationof meter 25. In Fig. 3, elements identical with those in` Fig. 2 have been given thesame reference characters. As shown, meter 25 is connected"v in the supply circuit of lamp it, being serially interposed betweenv conductor 22 and one terminal of Winding 43. As the energizing voltage of lamp HJ varies with the conductivity of control'tubeA whichis dependent` on the. anode voltage of'Y tube 3.5 the same relation existsasin'. the circuit of Figure 2 and the currentindication of meter 25 directly represents the density of sample i3 upon a uniformlyV With the described arrangements, an optical' feedz'backeiect is. produced in. which the energization'of the light source is varied as an inversefunctionr ofk the phototube output Current. This', in turn,.varies'the lamp intensity as an inverse logarithmic function of the phototube output. current. Consequently, logarithmic compensation useful in density` measurements is automatically accomplished so that a meter whose response is linear with respect to current may be used to directly indicate density upon a substantially uniformly graduated scale. Thus, a logarithmically responsive measuring circuit is provided incorporating a linear non-compensated amplifier Which need not be operated under such conditions as to have a logarithmic relation of grid potential to grid current.

It will be understood that other means of controlling the energizing current of the light source in response to the output of the phototube may be used. For example, a radio frequency oscillator circuit, such as commonly used in sound motion picture projectors for the exciter lamps, may be incorporated in the present arrangement. The essential requirement is that the intensity of the light source decreases in response to an increase in the flux incident upon the photo element, and that the overall gain of the amplier system is high.

An inherent characteristic of the described circuits is that, if the phototube-amplifier system has an extremely high gain, the amplifier system need not provide linear amplification. Thus, the amplification ratio may vary very widely over the operating range without adversely affecting the performance of the circuits.

While specific embodiments of the invention have been shown and described in detail to illustrate the application of the principles thereof, it will be understood that the invention may be otherwise embodied without departing from such principles.

What is claimed is:

1. An electrical measuring system for determining the density of translucent materials, comprising a photoelectric tube, a load resistance and a source of operating potential for said tube, an exciter light arranged to illuminate said photo tube through said translucent materials, said exciter light comprising a lamentary incandescent lamp having such characteristics that the light intensity emitted therefrom bears a logarithmic relation to the electrical energy supplied to said filament, a circuit for energizing said lament including a source of potential and a vacuum tube having anode and cathode electrodes connected effectively between said source and said filament and a grid electrode connected to said cathode through a grid resistor, the conductivity of said vacuum tube determining the current supplied to said filament and control means comprising an amplifier tube having an input circuit including said load resistance, and an output circuit including said grid resistor, whereby current variations in said output circuit control the conductivity of said vacuum tube in inverse relation to the response of said photo tube and the light intensity of said lamp is varied in inverse relation to the density of said material and a current indicating device connected in said filament circuit the indication of which may be marked directly in linearly spaced values of density.

2. An electrical measuring apparatus for directly indicating the photographic density of translucent sample materials, comprising a photoelectric tube, a source of operating potentials therefore, an exciter lamp arranged to illuminate said tube through samples which may be placed between said lamp and said tube, said lamp being of the filamentary incandescent type having such characteristics that the light intensity emitted therefrom bears a logarithmic relation to the electrical current supplied'toV the filament, a circuit for supplying current to said filament, electronic means of variable conductivity in said circuit for varying the magnitude of said lament current, electronic control means connected between said phototube and said filament current varying means operable to alter the conductivity thereof in inverse relation to the phototube current which is directly related to the light reaching said phototube upon a sample being placed between said lamp and said phototube whereby the light intensity of said lamp is varied in inverse relation to the density of said sample, and a current flow measuring means in said filament circuit the indication of which may be marked directly in linearly spaced density values.

3. An electrical measuring apparatus for directly indicating the photographic density of translucent sample materials, comprising a photoelectric tube, a source of operating potentials therefore, an exciter lamp arranged to illuminate said tube through samples which may be placed between said lamp and said tube, said lamp being of the lamentary incandescent type having such characteristics that the light intensity emitted therefrom bears a logarithmic relation to the electrical current supplied to the filament, a circuit for supplying current to said filament, including a vacuum tube the conductivity of which controls the magnitude of said lament current, electronic control means connected between said phototube and said vacuum tube for altering the conductivity thereof in inverse relation to the phototube current which is directly related to the light reaching said phototube upon a sample being placed between said lamp and said phototube whereby the light intensity of said lamp is varied in inverse relation to the density of said sample, and a current flow measuring means in said filament circuit the indication of which may be marked directly in linearly spaced values of density.

MONROE HAMILTON SWEET.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,881,336 Voigt Oct. 4, 1932 1,973,469 Denis Sept. 11, 1934 2,096,323 Gille Oct. 19, 1937 2,241,557 NicholsV May 13, 1941 2,241,743 Schoene May 13, 1941 2,245,124 Bonn June 10, 1941 2,413,706 Gunderson Jan. 7, 1947 

